All mammals exhibit a continuous process of protein turnover, involving both synthesis and degradation. Proteins may accumulate during growth, by enhanced rates of synthesis and/or reduced rates of degradation. The control of protein accumulation (i.e. growth) in the human fetus is subject to potential regulation by hormonal and nutritional factors within both material and fetal compartments, some of which may be unique to intrauterine development. We propose to study protein synthesis and degradation during late gestation in the fetal rat, with emphasis on skeletal muscle. both in vivo and in vitro techniques will be utilized to study protein turnover during normal development, and under circumstances of reduced maternal protein intake and fasting. The specific role of fetal insulin deficiency and exogenous insulin administration on fetal protein turnover will be evaluated. Branched chain amino acids, particularly leucine, have been demonstrated to enhance protein synthesis and inhibit protein degradation under certian experimental conditions. The metabolism of branched chain amino acids will be evaluated in fetal skeletal muscle, under normal conditions and as modified by nutritional and hormonal variants. Activities of enzymes involved in branched chain amino acid metabolism will be similarly studied in various fetal tissues. These studies should improve understanding of factors regulating protein turnover in the fetus and, as a corollary, of mechanisms controlling fetal growth.